The present invention relates to a brake booster negative pressure controller and, more particularly, to a brake booster negative pressure controller for use in an engine operated in a state where a suction negative pressure is low.
In recent years, a vehicle is often provided with a brake booster in order to reduce the stepping force when a brake pedal of the vehicle is stepped on. The brake booster is partitioned into two chambers A and B by a diaphragm therein. An assist force upon braking is obtained by the difference between pressures in the chambers. The chamber A as one of the two chambers is connected to an intake manifold of a suction system of an engine via a one-way valve by piping. When the negative pressure in the intake manifold is higher than that in the brake booster, the one-way valve is opened and the negative pressure in the intake manifold is introduced into the chamber A. In a state where the brake is not applied, the pressures in the two chambers are equal. When the brake is applied, atmosphere is introduced into one of the chambers, causing the pressure difference between the two chambers, and the assist force upon braking is consequently obtained.
In lean-burn engine, cylinder injection engine, or the like, however, operation is performed while taking a large volume of new air, so that only a negative pressure lower than a conventional one is obtained in the intake manifold. Consequently, the negative pressure in the brake booster cannot be sufficiently increased and a sufficient assist force cannot be obtained upon braking.
In order to solve such a problem, the technique (refer to Japanese Patent Application Laid-Open No. 7-247866) of always monitoring the pressure in the brake booster, closing a throttle valve by a predetermined amount while the pressure value is on the atmosphere side more than a predetermined threshold, and increasing the negative pressure in the intake manifold has been proposed.
On the other hand, there is provided the technique (refer to Japanese Patent Application Laid-Open No. 8-164840) of a brake booster negative pressure controller comprising: pressure sensing means for sensing a pressure acting on a brake booster; throttle valve closing means for closing the throttle valve by a predetermined amount when a pressure sensed by the pressure sensing means is lowered below a predetermined threshold pressure; brake operation sensing means for sensing the operation of the brake; and throttle valve closing means for closing the throttle valve by a predetermined amount in the case where the pressure sensed by the pressure sensing means is lowered below the predetermined threshold pressure when the brake is operated by brake operating means.
The braking performance required of the brake is largely influenced by the driving state of the vehicle. When the speed is high, the higher braking performance is necessary. The above-mentioned two techniques of the negative pressure controller intend to always obtain the brake booster negative pressure of a predetermined amount or more at an arbitrary speed. As a result, sufficient braking performance can be always obtained at an arbitrary speed of the vehicle. For this purpose, however, the negative pressure threshold has to be determined on the basis of a case where the highest braking performance is required (at the time of high speed of the vehicle or the like) as a reference.
Consequently, even when the sufficient braking performance is assured at the time of low speed (sufficient negative pressure is assured in the brake booster), the throttle valve is closed by a predetermined amount at the time point when the negative pressure in the brake booster becomes lower than the threshold. Because of the useless operation, there are problems that pumping loss of the engine is increased and the fuel consumption deteriorates.
When the brake is applied while the accel is stepped on, the braking performance higher than that in an ordinary state is required since the braking is already applied. In the conventional brake booster negative pressure controller, however, no measure is taken against the case.
Further, when the negative pressure in the brake booster cannot be normally sensed due to a failure in a sensor or the like, even if the negative pressure in the brake booster is insufficient, there is a case that such a state cannot be detected. There is consequently the possibility that the sufficient braking performance cannot be obtained.